Photosensitive coating material for a substrate

ABSTRACT

A radiation-sensitive coating material, in addition to a base polymer, has a solvent and a radiation-active substance which forms an acid on irradiation by light (including energetic electrons or ions), a fluorescent substance which alters its fluorescence property subject to a change in the acid content of its surroundings. In a process for exposing a substrate coated with the coating material at least one sensor in the exposure chamber of the exposure apparatus measures the intensity of the change in fluorescence spectrum as a function of time during the exposure operation. From the course of intensity at the time of an individual line of the fluorescence spectrum or the intensity integrated over a wavelength interval it is possible to determine the endpoint of the exposure operation by way of electronic algorithms. Deviations from experimentally determined ideal curves of the intensity course provide information on erroneous functions in the course of coating material application and exposure.

This is a divisional application of application Ser. No. 10/673,964,filed Sep. 29, 2003; the application also claims the priority, under 35U.S.C. §119, of German patent application DE 102 45 128.1, filed Sep.27, 2002; the prior applications are herewith incorporated by referencein their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a photosensitive coating material forcoating a substrate.

In the fabrication of integrated circuits a lithographic projectionprocess is used to transpose patterns from a mask onto a substrate: asemiconductor wafer or a flat panel, for instance. Transposition takesplace into a layer comprising a photosensitive coating material(resist). After the exposed parts of the coating material (positiveresist) have been developed and removed the patterned resist layeritself is utilized as a mask for transposing the pattern into anunderlying layer by way, for example, of an etching process.

To transpose the mask pattern into the resist layer it is possible touse photon radiation or particle radiation. The wavelengths normallyused in this case are located in the visible optical range, in theultraviolet range (DUV, deep ultraviolet, and VUV, vacuum ultraviolet)or in the soft X-ray range, which is also called EUV (extremeultraviolet). Exposure can also be carried out by particle lithography,an example of which is ion projection lithography (IPL). The use ofelectron beams (EBL), which is already known from mask exposure, is afurther possibility. The particle energies or corpuscular wavelengthsdepend here on the acceleration voltages used, typically from 30 to 100keV.

In the case of the positive resists whose use is presently preferred onerequirement is to set the radiation dose for the exposure such that thepattern is transposed into the photosensitive layer true to scale andthat the line profiles formed in the photosensitive layer followingdevelopment exhibit a high steepness.

An inadequate exposure dose with beams or particles can lead to anincomplete removal of the resist in the exposed areas, so that in thecase of the positive resist linear distances are formed with too smallan extent, if at all. In the case of a subsequent etching operation,flat resist profiles can lead to an uncontrollable transposition of thepattern widths of lines.

This problem also occurs in the case of overexposure with too great anexposure dose, wherein case the exposed areas are undesirably widened.

The factors which determine an exposure dose include the following: theexposure power of the radiation, the exposure time, the thickness of thephotosensitive layer, the chemical composition and sensitivity of thephotosensitive layer, the optical properties of underlying layers, suchas their reflectivity, for example, and also charging effects of theexposed patterns, etc.

Owing to the very low pattern widths to be achieved on the substrate itis generally insufficient to define theoretically the optimum exposuredose for true-to-scale imaging. A role is also played, for example, byproblems such as nonuniform illumination of the exposure field as aresult of deadjustment of the optical system or of the illuminator,deterioration in the optical or ion-optical components, fluctuations inthe light source or particle source, external mechanical orelectromagnetic interference, which in particular may also occursuddenly in each case. As regards the thickness of the photosensitivelayer it is also possible for center/edge variations, caused during thespin coating of the photosensitive resist, or else for the underlyingpattern topography to play a part. The chemical properties of the resistmay likewise vary over time, so that two successive substrates to beexposed, with the same resist, are subject to different resistsensitivities. One example is the storage time of the resist-coatedsubstrate, particularly in the case of chemically amplified resists(CARs).

For a batch of, say, 25 substrates it is therefore usual to select whatis termed a precursor substrate and to expose and develop it withdifferent exposure doses, known as the exposure scale. The exposedpatterns are subsequently measured for their pattern width and comparedwith the respective target pattern width. The exposure dose allocated tothe pattern with the greatest match is then used to expose the remaining24 substrates of the batch.

A disadvantage with this process is that it is time-consuming and at thesame time the capacity of the exposure apparatus in question is notbeing fully utilized to fabricate saleable end products. There is aconsequent increase in the costs of fabricating a product. Furthermore,variations in the above-mentioned factors that occur during theproduction sequence of one batch or else of just a single substratecannot be taken into account by altering the exposure dose accordingly.

More recently, therefore, exposure apparatus has been developed whereinat least the variations in exposure output caused by the exposureapparatus itself can be corrected by means of a measurement of preciselythe output in the region of the substrate. Corresponding controlmechanisms are made available via the actuation of shutters, diaphragmsand/or the speed of the scanning platforms in the case of scanners.

Such control mechanisms can only be used, however, with exposureapparatus wherein the exposure operation is conducted in the visibleoptical range or in the ultraviolet range. In the case of extremeultraviolet radiation (EUV) the beam splitters which couple out thelight to be measured at the location of the substrate would no longer betransparent and would cause at least partial shading of the exposurefield.

In that kind of exposure apparatus, therefore, the exposure output ismeasured at the location of the light source, and not the substrate, andis integrated over time to give an exposure dose. When a desired targetlevel has been reached, exposure is ended by means of the controlmechanisms. The fraction of radiation actually incident on the substrateis determined beforehand and later on is merely combined with theradiation dose determined at the location of the light source in thecalculation of the exposure output actually determined.

This calculation does not take account of variations from material tomaterial, or for a relatively short time period, as a result of furthermirror elements between the light source and the substrate, as also theimaging optical system, the wavelength filters, the vacuum windows orthe EUV reflection mask. Slower variations can again be calibrated outby means of precursors. Changes which occur over very short timescales,on the other hand, lead to erroneous exposures in the photosensitivelayer on the substrate. The projection systems described at the outsetwith regard to particle lithography are subject to similar problems.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a photosensitivelacquer and an exposure apparatus, which overcomes the above-mentioneddisadvantages of the heretofore-known devices and methods of thisgeneral type and which renders it possible to adjust an exposure dose insuch a way that patterns can be transposed true-to-scale into thephotosensitive layer in a lithographic projection step.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a photosensitive coating material forcoating a substrate, comprising:

a base polymer, a solvent, a photoactive substance that forms an acidupon irradiation with light; and

a fluorescent substance surrounded by at least one material and having afluorescence property, the fluorescence property of the fluorescentsubstance changing in dependence on an acid fraction present in the atleast one material.

With the above and other objects in view there is also provided, inaccordance with the invention, a process for exposing a substrate in anexposure apparatus, which comprises the steps of:

providing a substrate coated with the radiation-sensitive coatingmaterial as outlined above;

loading the substrate into an exposure apparatus having at least onefirst sensor for detecting light re-emitted by the fluorescent substancein the coating material within a first wavelength range;

starting a first exposure operation by projecting a pattern into thephotosensitive layer;

firstly measuring a first intensity of light emitted by the fluorescentsubstance at a first point in time with the at least one first sensor;

at least once secondly measuring a second intensity of light emitted bythe fluorescent substance at at least one second point in time, with theat least one first sensor;

comparing the first intensity with the second intensity; and

ending the exposure operation in dependence on a comparison result ofthe comparing step.

In accordance with the present invention a photosensitive coatingmaterial for coating a substrate for conducting an exposure operationcomprises a base polymer, a solvent, a photoactive substance, and afluorescent substance. The light emitted by the fluorescent substanceduring an exposure operation can advantageously be analyzed in order toprovide information on the progress of the exposure operation, asdescribed in the following text:

The photoactive substance possesses the property of forming an acid onirradiation by light, for example, during an exposure operation. Opticalphotoresists used conventionally, but also those of the ultravioletlight range (DUV, VUV), such as the chemically amplified photoresists(CARs) used since recent times, comprise such photoactive substanceswherein an acid is formed in this way.

In the case of the photoresists which operate in the visible optical andultraviolet range this photoactive substance is, for example,diazonaphthoquinone, which on exposure to light in the presence ofmoisture (H₂O) is converted into an acid, namely the carboxylic acid. Aphotochemical reaction of this kind is advantageous, for example, at thewavelengths 436 nm (g-line) or 365 nm (i-line). The carboxylic acidliberated separates the matrix consisting of a novolak as base polymer,so that the regions of the resist that were previously exposed can nowbe removed in a developer step.

In the case of the chemically amplified photoresists the photoactivesubstance likewise brings about, in a photochemical reaction, theformation of an acid which acts catalytically to produce further acidmolecules. This process, however, takes place only in a heating stepdownstream of exposure, the so-called post-exposure bake operation.Constituents of the polymer chains of the base polymer that werepreviously insoluble are converted into soluble components in a chainreaction in this operation. These resists, which are normally used at248 nm (DUV) and 197 nm (VUV), can also be used successfully in EUVlithography.

In both cases the presence and the number of acid molecules formedlocally constitute a measure of the progress of the exposure operation.One way to obtain information on this measure is—in accordance with theinvention—to use at least one fluorescent substance whose fluorescenceproperty depends on the fraction of acid formed at a particular momentin time. As exposure progresses, and hence as the acid contentincreases, there is a decrease in the absorption coefficient of theresist, so that increasing numbers of fluorescent molecules are excitedand the fluorescence intensity rises. The light re-emitted by thefluorescent substance can be captured with a sensor and evaluated. Thefluorescence property is reflected in the spectrum and/or in theintensity of the light re-emitted by the fluorescent substance.

As fluorescent substances it is possible in particular to use what arecalled fluorescence indicators, which exhibit defined changes in theemitted fluorescent radiation when the pH changes. Fluorescenceindicators of this kind are known, for example, from “Handbook ofChemistry and Physics”, 55th edition, 1974-1975, CRC Press, p.D-117-D-118, which provides in particular a table of fluorescentsubstances in particular for the titration of opaque, cloudy or highlycolored solutions. From this mention may be made by way of example ofbenzoflavine, which at a pH of 1.7 and above has a spectral course whichis assigned a color value “green”, and which at a pH of from 1.7 to 0.3has a spectral course which is assigned a color value “yellow”. In otherwords, when there is a relatively high fraction of acid in thephotosensitive coating material, the fluorescent emission spectrum inthe course of exposure to light is shifted to longer wavelengths. Thecolor change occurs at a particular pH, in this case 1.7.

If, then, the pH falls below a value which is dependent on theparticular substance (in the above case, a pH of 1.7), there is,accordingly, a color change; the wavelength of the radiation emitted isaltered. The intensity of the radiation emitted by the fluorescentsubstance increases in proportion with the amount of acid liberatedduring exposure and with the associated decreasing absorption of theresist and undergoes the transition to a saturation phase as soon as thephotosensitive coating material is fully exposed.

By measuring the change in intensity of the emitted fluorescentradiation over time it is therefore possible to monitor the exposureoperation in situ, thereby making it possible in accordance with theinvention to recognize the endpoint of the exposure operation. Thetransition of the substantially proportional increase in intensity tothe saturation phase corresponds precisely to this endpoint.

In the process of the invention, therefore, during the exposureoperation in the exposure apparatus, one or more sensors are used todetect the light emitted by the fluorescent substance. These sensors arepreferably arranged in the exposure chamber of an exposure apparatus.

Where the sensors cover only a limited wavelength range, it is preferredto use at least two sensors whose sensitive wavelength ranges aredifferent. In this way it is possible to record at least a section ofthe spectrum of the emitted light, thereby allowing assignment to a pHon the basis, for example, of a table held in the measurement andevaluation unit.

Also envisaged is the combination of different fluorescent substances inthe photosensitive coating material, in order to allow the pH to bedetermined over relatively wide ranges.

Instead of a particular threshold value for determining entry into thesaturation phase it is also possible to analyze the course of the curve.In the first case the fraction of the area exposed would have to berelated to the measured intensity. In the second case it would bepossible, for example, to detect the inflection point of the curve as itentered the saturation phase.

The invention accordingly envisages using fluorescent substances whichallow either the measurement of fluorescence intensities emittedproportionally to the pH or the determination merely of the colorchanges as a function of the acid fraction; in the first case all thatis necessary is to determine the intensity of at least one wavelengthrange, while in the second case it is necessary to measure at least twowavelength ranges by means of sensors.

A further possibility is to use a fluorescent substance and to analyzeits light without said property of dependence on the acid content of thesurrounding material. Indeed, if exposure is carried out using a lightbeam which as time progresses generates an increasingly deeper exposureprofile in the coating material, so that at the site of the exposurethis coating material becomes more and more transparent for thefluorescence intensity, then here as well, on entry into saturation, thecoating material can be assumed to undergo satisfactory full exposure.For this purpose it is preferred to utilize a second light source, byvirtue of the process of the invention, in addition to the exposure beamfor forming the pattern, in order to allow the fluorescent substance tobe excited uniformly as time progresses.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a photosensitive coating material for a substrate, it is neverthelessnot intended to be limited to the details shown, since variousmodifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational diagram showing an exemplary embodiment of aphotosensitive coating material according to the present invention,which is exposed in an exposure apparatus; and

FIG. 2 is a graph plotting the time course of the intensity of theradiation re-emitted by the fluorescent substance in accordance with oneexemplary embodiment of the process according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown an exemplary embodimentof the present invention, in a diagrammatic illustration. A substrate10—in this case a semiconductor wafer—is coated with a photosensitiveresist 20. In the exemplary embodiment the exposure beam is an electronbeam 1, which exposes a pattern in the photosensitive resist 20. Theresist is sensitive to irradiation with electrons (and in this documentis referred to as “photosensitive”). The semiconductor wafer 10 is firstloaded into an exposure apparatus having an exposure chamber. Theexposure chamber has a sensor 30 which is able to detect the intensityof emitted radiation in at least two wavelength ranges. The sensorsystem may also comprise a plurality of sensors 30, two for example,each of which is able to receive one wavelength range. The use of anoptical grating with a diode array that covers the entire spectrum ofinterest with a corresponding wavelength range can likewise be realized.

With reference to FIG. 2, the exposure operation is started at a timet=0. At this point in time there are as yet no acid fractions in thephotosensitive resist. The pH is approximately 7.0. As time progresses,acid groups are formed in a photochemical reaction, so that the pHfalls. When a certain pH of, say, 6.4 is reached the color value of thespectrum of radiation emitted by the fluorescent substance begins tochange. Whereas in the first wavelength range of the sensor 30 theintensity falls, the intensity in the second wavelength range begins torise. FIG. 2 shows only the intensity course 101 of the secondwavelength range. On reaching a pH of, say, 5.0 the photosensitiveresist 20 is fully exposed. Acids can no longer be formed, so that inthe time course a saturation profile is established.

When saturation has been achieved, i.e., when no further increase isfound in the measured intensity, then in accordance with the process ofthe invention the exposure operation is ended as a reaction to thisevent. Further exposure would expose only adjacent resist areas andwould lead to a deleterious widening of the line profile.

In accordance with the invention, however, the exposure operation can beended before saturation is reached if the instantaneous color value andhence also the instantaneous pH is determined from the comparison of themeasurements of the two sensors 30. If a predetermined target for the pHin this respect is known, then known control mechanisms can be used toadvance a shutter or to switch off the light source when this target isreached. A prerequisite here is the holding of a table with anassignment of color value and wave-length.

If the course of an ideal intensity curve 101 is known, then from anumber of individual measurements leading to a second intensity curve102 (FIG. 2) it is possible to conclude the presence of further problemfactors not associated with the variation of the light source over time.The size and shape of the area 200 in FIG. 2, as a measure of thedistance to the ideal curve course 101, gives indications of whether,for example, there are inhomogeneities in the resist thickness, in theillumination of the exposure field, or in the under-lying patterntopography. In this case some areas of the resist will reach theirsaturation profile before other resist areas, so that the recordedsaturation is established in a time interval which is dependent on theinhomogeneities.

Advantageously, by virtue of the present invention, the exposure dosescan be set individually for each individual exposure field even within asubstrate or semiconductor wafer in a manner optimized for atrue-to-scale reproduction of image. The yield is thereby increased.Moreover, production costs are reduced, since there is no need to runprecursor substrates. Where deviations from an experimentally determinedideal intensity curve 101 are ascertained in the course of exposure, animmediate apparatus monitoring operation can be initiated in reaction tothe deviations found from a corresponding comparison of the curves,thereby preventing excessive production of reject products before theproblem is ascertained in another way.

In the first exemplary embodiment, acid molecules are formed over theentire thickness range of the resist from time t=0, since the electronbeam normally exhibits high depths of penetration. The increase in theintensity of the fluorescent light in one wave-length range, or thedecrease in the other wavelength range, is therefore initiallyproportional to the progressing time.

In a second exemplary embodiment a laser beam is used for exposure at awavelength of 365 nm. As time progresses an exposure profile isdeveloped within the resist, since owing to absorption in the resistfull exposure does not take place from the outset. In this case anidealized intensity curve may differ consider-ably from a linearcorrelation with time.

In the above examples the electron, ion or light beam used to expose theresist was also used to excite the fluorescent light. A furtherexemplary embodiment envisages installing a further illumination sourcewhich allows the color molecules formed by the electron, ion or lightbeam to be excited. The change in these fluorescence intensities overtime as a function of the exposure dose reflects the status of theexposure operation. The further illumination source should in this casebe provided in such a way that the resist itself is not impairedphotochemically, i.e., exposed, something which can be realized by wayof a suitable wavelength for the illumination source, differingconsiderably from 365 nm, for example.

1. A photosensitive coating material for coating a substrate,comprising: a base polymer; a solvent; a photoactive substance formingan acid upon irradiation with light; and a fluorescent substancesurrounded by at least one material and having a fluorescence property,the fluorescence property of said fluorescent substance changing independence on an acid fraction present in said at least one material. 2.The photosensitive coating material according to claim 1, wherein thefluorescence property designates a spectrum of the light re-emitted bysaid fluorescent substance.
 3. The photosensitive coating materialaccording to claim 1, wherein the fluorescence property designates anintensity of the light re-emitted by said fluorescent substance.
 4. Thephotosensitive coating material according to claim 1 implemented as achemically amplified coating material.
 5. The photosensitive coatingmaterial according to claim 1 configured as a coating material sensitivefor wavelengths of less than 157 nanometers.
 6. The photosensitivecoating material according to claim 1 formed with molecular chains, andwherein said photoactive substance and said fluorescent substance areparts of common molecular chains.
 7. A radiation-sensitive coatingmaterial for coating a substrate, comprising: a base polymer; a solvent;a radiation-active substance forming an acid upon irradiation withenergetic radiation; and a fluorescent substance having a fluorescenceproperty, said fluorescent substance changing a fluorescence independence on an acid fraction present in a material surrounding saidfluorescent substance.